In clinical use, infusion pumps allow automatically infusing liquid drugs or nutrients to patients intravenously at a set speed within a relatively longer time, which reduces the workload of medical staff and improves the security of infusion. According to different working principles, the infusion pumps are divided into different types, among which capacity pumps and syringe pumps are common. The capacity pumps can be driven by peristaltic pumps; and the syringe pumps can be driven by lead screw pumps. A motor drives a lead screw to rotate, changes rotational motion into linear motion, and pushes a plunger to inject liquid in a syringe into a patient's vein through a nut connected with the plunger. The precision of an infusion speed of the syringe pumps is higher than that of the capacity pumps, of which an error is about ±5%. However, in some clinical applications, as intravenous injection for new babies and insulin injection for diabetic patients, it is necessary to further improve the precision of the infusion speed of the syringe pumps. During infusion, it is also necessary to know the position of the plunger in the syringe. A common practice is to use a step motor, and it is also feasible to use a linear or rotating potentiometer or an optical decoder to control the infusion speed, but these methods still have problems of low precision, poor reliability and high manufacturing cost.